For a conventional single-stage AC-DC converter with high power factor, the output voltage usually contains a low frequency (double line frequency, or second harmonic) ripple component, due to imbalance of input and output power. This second harmonic (e.g., 120 Hz in North America or 100 Hz in China, Europe) is of particular concern for DC lighting applications, such as LED lighting, as it results in visible flickering wherein the human eye can see fluctuation of the light emitting from the LED. The fluctuating light output may be undesirable in certain lighting applications, as well as harmful to human eyes.
In order to solve this problem, a series ripple cancellation converter (RCC) may be used to cancel the double line frequency voltage ripple from the single-stage LED driver. The RCC may be an additional small power converter that is connected in series with the main power factor correction (PFC) output. As a result, a pure DC voltage is obtained and is applied to the LED lamps to produce DC LED current.
Series ripple cancellation converters typically employ a voltage-ripple-based feedback control strategy, as shown in the block diagram of FIG. 1A. According to this approach, the two series-connected output voltages (the main PFC stage output voltage, νmain, and the RCC stage output voltage, νFB) are sensed simultaneously to achieve ripple cancellation. The sensing circuit for the RCC is shown in FIG. 1B where two differential to single-ended voltage conversion circuits are required. Thus, series ripple cancellation converters that employ a voltage sensing feedback control strategy suffer from the drawback of relatively complex and uneconomical signal-sensing circuits. Moreover, the voltage sensing feedback control strategy has a potential mismatch problem due to parameter tolerance of the sensing circuits, resulting in inferior ripple cancellation performance.